Optimization of microionization chambers for small-field reference dosimetry

Transcription

1 Optimization of microionization chambers for small-field reference dosimetry Jessica R. Snow University of Wisconsin-Madison Department of Medical Physics Medical Radiation Research Center NCCAAPM Fall Meeting October 0 th, 203

2 Ionization chambers The ionization chamber is the most commonly used device for radiation therapy measurements Provide a high level of precision and accuracy Provide near independence of beam energy, dose, and dose-rate Task Group 55 relies on the response of an ionization chamber to provide small and composite field dosimetry traceable to broad-beam calibration 2

3 Microionization chambers Small fields require detectors with high spatial resolution Standard ionization chambers are often too large for small-field dosimetry Small-volume ionization chambers were developed, called microchambers 3 Image source:

4 Outline Small-field dosimetry with microchambers Current state of microchamber dosimetry Optimization of microchamber design 4

5 Microionization chambers Microionization chambers: measuring volume 0.02 cm 3 Inner diameter ranging from 2 mm to 4 mm Length ranging from 2 mm to 5 mm

6 Small and nonstandard fields Plan-class specific reference field: D f pcsr w,q pcsr f = M pcsr f Qpcsr N D,w,Q0 k Q,Q0 k pcsr, f ref Qpcsr,Q polarity correction factor fully corrected chamber reading M = M raw P TP P pol P ion P elec Machine-specific reference field: ion chamber recombination reading correction factor D f msr w,q msr f = M msr f Qmsr N D,w,Q0 k Q,Q0 k msr, f ref Qmsr,Q Spectrum of a radiation beam changes with field size 6

7 Reference-class ionization chambers. Polarity correction should be <0.4% and vary <0.5% over the photon energy range of interest ( 60 Co to 25 MV) 2. The inverse of the chamber response should vary linearly with the inverse of the applied voltage 3. Ion recombination correction factor should vary linearly with the dose per pulse (D pp ) with an intercept of < Initial recombination for opposite polarities should agree to within 0.%. M. R. McEwen. Measurement of ionization chamber absorbed dose k Q factors in megavoltage photon beams. Med. Phys., 37:

8 Farmer-type chamber: P pol and P ion normalized chamber response Saturation curve: Exradin A2 negative charge collection positive charge collection applied voltage (V) /(normalized chamber response ) Jaffé plot: Exradin A2 negative charge collection positive charge collection fit (negative charge collection) fit (positive charge collection) /(applied voltage) (V - ) 8

9 Farmer-type chamber: P ion vsd pp normalized chamber response Saturation curve: Exradin A mgy/pulse 0.27 mgy/pulse 0.28 mgy/pulse 0.38 mgy/pulse 0.40 mgy/pulse applied voltage (V) /(normalized chamber response) Jaffé plot: Exradin A mgy/pulse 0.27 mgy/pulse 0.28 mgy/pulse 0.38 mgy/pulse 0.40 mgy/pulse /(applied voltage) (V - ) 9

10 Farmer-type chamber: P ion vsd pp Exradin A2.005 P ion (300 V) negative charge collection positive charge collection fit (negative charge collection) fit (positive charge collection) D pp (mgy) ( P ion =+ γ +δd ) w,pp V 0

11 Microchambers Smaller collecting volume = smaller signal Exradin A2 (0.65 cm 3 ) ~ 90 times smaller volume Exradin A6 (0.007 cm 3 ) ~ 90 times smaller signal Lower signal to noise ratio Increased sensitivity to typical ionization chamber issues Image source:

12 Microchamber: Saturation curves normalized chamber response PTW TN304 negative charge collection positive charge collection applied voltage (V) normalized chamber response Exradin A4SL negative charge collection positive charge collection applied voltage (V) 2

13 Microchamber: Jafféplots /(normalized chamber response ) PTW TN304 negative charge collection positive charge collection fit (negative charge collection) fit (positive charge collection) /(applied voltage )(V - ) /(normalized chamber response ) Exradin A4SL negative charge collection positive charge collection fit (negative charge collection) fit (positive charge collection) /(applied voltage )(V - ) 3

14 Microchamber: P ion vsd pp normalized chamber response Saturation curve: Exradin A4SL 0.00 mgy/pulse 0.27 mgy/pulse 0.38 mgy/pulse 0.40 mgy/pulse applied voltage (V) /(normalized chamber response) Jaffé plot: Exradin A4SL 0.00 mgy/pulse 0.27 mgy/pulse 0.38 mgy/pulse 0.40 mgy/pulse fit /(applied voltage) (V - ) 4

15 Microchamber: P ion vsd pp Exradin A4SL P ion (300 V) negative charge collection positive charge collection fit (negative charge collection) fit (positive charge collection) D pp (mgy) P ion =+ γ +δd w,pp ( ) V 5

16 Energy dependence 6

17 Microchamber behavior Large voltage-dependent effects Anomalous ion recombination correction factors Significant energy dependence Variations of 0%-70% in N k for medium-energy x-ray beams relative to 60 Co 7

18 Microchamber prototype Manufactured four low-z microchambers Collecting volume of 0.05 cm 3 Shell, guard, and collecting electrode composed of a low-z conductive plastic Low-Z conductive plastic 8

19 Microchamber prototypes normalized chamber response Prototype # positive charge collection negative charge collection normalized chamber response Prototype # 2 positive charge collection negative charge collection applied voltage (V) applied voltage (V) normalized chamber response Prototype # 3 positive charge collection negative charge collection normalized chamber response Prototype # 4 positive charge collection negative charge collection applied voltage (V) applied voltage (V)

20 Behavior isolation Investigated variables: Stem/cable irradiations Assembly Contaminants Electric field lines High-Z materials 20

21 Behavior isolation normalized chamber response.05 Isolate the behavior to a chamber component Shell electrode HV insulator Guard electrode Guard-collector insulator Collecting electrode Prototype # 2 positive charge collection negative charge collection normalized chamber response Prototype # 4 Voltage-dependent polarity effects positive charge collection negative charge collection applied voltage (V) applied voltage (V) 2

22 original assembly internal components swapped original assembly guard swapped original assembly collector swapped guard/coll. insulator swapped Behavior isolation Prototype # 4 2 Prototype 4 Prototype 2 positive charge collection negative charge collection applied voltage (V) applied voltage (V) 22 Percent difference (I300 V, I 25 V ) [%] normalized chamber response

23 Electrode conductance normalized chamber response Prototype # 3 Potential difference between the bias of the 0.98 guard and collecting electrode causing a distortion of 0.97 the electric field lines Original Graphite coated guard Negative Positive charge collection Positive Negative charge collection applied voltage (V) 23

24 COMSOL Multiphysics simulations COMSOL Multiphysics software Collector: 300 V Guard: 300 V 24

25 Volume simulations Collector: 300 V Guard: 290 V Collector: 300 V Guard: 300 V Collector: 300 V Guard: 30 V 25

26 Volume simulations Collector: 300 V Guard: 290 V Collector: 300 V Guard: 300 V Collector: 300 V Guard: 30 V 26

27 Volume simulations 20 0 Microchamber (0.08 cm 3 ) Farmer-type chamber (0.650 cm 3 ) Vol diff (%) Bias diff (%) 27

28 Energy dependence Manufactured a low-z microchamber (volume = 0.04 cm 3 ) Shell, guard, and collecting electrode composed of a low-z conductive plastic Low-Z conductive plastic Coated the collecting electrode with a high-z silver epoxy (Z=47) High-Z silver epoxy 28

29 Energy dependence.2. M20 M50 M00 M50 M250 with silver without silver N K normalized

30 Optimized microchamber design Manufactured three low-z microchambers Collecting volume of 0.08 cm 3 Shell, guard, and collecting electrode composed of a low-z conductive plastic Low-Z conductive plastic 30

31 Optimized design: Energy dependence.6 M20 M50 M00 M50 M200 M250 normalized N K Prototype Prototype 2 Prototype effective energy (kev)

32 Optimized design: Saturation curves normalized chamber response J2 prototype negative charge collection positive charge collection normalized chamber response J2 prototype 2 negative charge collection positive charge collection applied voltage (V) applied voltage (V) 32

33 Optimized design: Saturation curves normalized chamber response J2 prototype 3 negative charge collection positive charge collection applied voltage (V) 33

34 Conclusions Need to ensure that microchambers meet the reference-class requirements Commercial chambers exhibit Voltage-dependent polarity effects Anomalous ion recombination correction factors Significant energy dependence Reduce voltage-dependent polarity effects by eliminating the potential difference between the guard and collecting electrode Reduce energy dependence by eliminating high-z materials 34

35 Future Work Continue to improve microchamber design Low-Z Eliminate potential difference between the electrodes Characterize the optimized chambers for all reference-class requirements Integrate into small and nonstandard dosimetry 35

36 Acknowledgements Dr. Larry DeWerd Brian Hooten Standard Imaging staff and students John Micka Dr. Steve Davis Frank Grenzow Ben Palmer Dr. Malcolm McEwen - NRC UWADCL customers 36